Hydrocarbons are essential in modern life. Hydrocarbons are used as fuel and raw material in various fields, including the chemical, petrochemical, plastics, and rubber industries. Fossil fuels, such as coal, oil and natural gas, are composed of hydrocarbons with varying ratios of carbon to hydrogen. Despite their wide application and high demand, fossil fuels also have limitations and disadvantages, particularly due to their finite reserve, irreversible combustion and contribution to air pollution (and thus to global warming). Regardless of these problems the more efficient use of still existing natural gas sources is highly desirable. Further new sources and ways for recyclable and environmentally benign carbon fuels are needed.
One alternative frequently mentioned non-carbon fuel is hydrogen, and its use in the so-called “hydrogen economy.” Hydrogen is thought to be beneficial as a clean fuel, producing only water when combusted. Free hydrogen, however, is not a natural primary energy source on earth, due to its incompatibility with the atmospheric oxygen. It must be generated from hydrocarbons or water, which is a highly energy-consuming process. Further, as hydrogen is produced from hydrocarbons or coal, any claimed benefit of hydrogen as a clean fuel is outweighed by the fact that its generation, mainly by reforming of natural gas, oil or coal to synthesis gas (“syn-gas” a mixture of CO and H2), or the generation of electricity for the electrolysis of water is far from clean, besides hydrogen is difficult and costly to handle, transport and distribute. As it is extremely light, volatile and potentially explosive, it requires high-pressure equipment. The needed non-existent infrastructure also necessitates special materials to minimize diffusion and leakage, and extensive safety precautions to prevent explosions.
The continued importation of natural gas from far away and frequently difficult to access locations also necessitates its safe storage and transportation particularly when involving liquefied natural gas (LNG). This necessitates transporting LNG at low temperatures in its liquid form exposing it to serious environmental and safety hazards. It is suggested that a more practical and safe alternative for LNG is methanol, or dimethyl ether (DME), which are readily produced from natural gas. Methanol is the simplest liquid oxygenated hydrocarbon, differing from methane (CH4) by a single additional oxygen atom. Methanol, also called methyl alcohol or wood alcohol, is a colorless, water-soluble liquid with a mild alcoholic odor. It is easy to store and transport. It freezes at −97.6° C., boils at 64.6° C., and has a density of 0.791 at 20° C.
Methanol is a convenient safe liquid easily obtained from existing coal or natural gas sources via methods developed and practiced since the 1920's. However, these methods using conversion (reforming) of coal and subsequently natural gas to syn-gas (a mixture of H2 and CO) are highly energy consuming and produce large amount of CO2 as a by-product. This is notably an economic disadvantage but also represents a serious environmental problem by increasing one of the main greenhouse gas causing global warming.
Methanol not only represent a convenient and safe way to store and transport energy, but together with its derived product dimethyl ether (DME), is an excellent fuel. Dimethyl ether is easily obtained from methanol by dehydration or from methane (natural gas) with CO2 via a bi-reforming process. It is a particularly effective fuel for diesel engines because of its high cetane number and favorable combustion properties. Methanol and dimethyl ether exceedingly blend well with gasoline or diesel oil to be used as fuels in internal combustion engines or electricity generators. One of the most efficient use of methanol is in fuel cells, particularly in direct methanol fuel cells (DMFC), in which methanol is directly oxidized with air to carbon dioxide and water while producing electricity.
Contrary to gasoline, which is a complex mixture of many different hydrocarbons and additives, methanol is a single simple chemical compound. It contains about half the energy density of gasoline, meaning that two liters of methanol provide the same energy as a liter of gasoline. Even though the energy content of methanol is lower, it has a higher octane rating of 100 (average of the research octane number (RON) of 107 and motor octane number (MON) of 92), which means that the fuel/air mixture can be compressed to a smaller volume before being ignited. This allows the engine to run at a higher compression ratio of 10-11 to 1 more efficiently than the 8-9 to 1 ratio of a gasoline-powered engine. Efficiency is also increased by methanol's (oxygenates') higher “flame speed,” which enables faster, more complete fuel combustion in the engines. These factors explain the high efficiency of methanol despite its lower energy density than gasoline. Further, to render methanol more ignitable even under the most frigid conditions, methanol is mixed with gasoline, and other volatile components or with a device to vaporize or atomize methanol. For example, an effective automotive fuel comprised by adding methanol to gasoline with the fuel having a minimum gasoline content of at least 15% by volume (M85 fuel) so that the engine can readily start even in low temperature environments were commercially used in the US in the 1980's. M20fuel (with 20 volume % methanol) is also being introduced. Similarly, dimethyl ether (DME) mixed with diesel oil or in household use as a substitute of natural gas or LPG is of commercial interest. These mixtures are not only efficient fuels but conserve or replace decreasing oil resources. The amount of methanol or dimethyl ether added can be determined depending upon the specific condition and needs.
Methanol has a latent heat of vaporization of about 3.7 times higher than gasoline, and can absorb a significantly larger amount of heat when passing from liquid to gaseous state. This helps to remove heat away from the engine and enables the use of an air-cooled radiator instead of a heavier water-cooled system. Thus, compared to a gasoline-powered car, a methanol-powered engine provides a smaller, lighter engine block, reduced cooling requirements, and better acceleration and mileage capabilities. Methanol and DME are also more environmentally-friendly than gasoline or diesel oil, and produce low overall emissions of air pollutants such as certain hydrocarbons, NOx, SO2 and particulates.
Methanol is also one of the safest fuels available. Compared to gasoline, its physical and chemical properties significantly reduce the risk of fire. Methanol has lower volatility, and methanol vapor must be four times more concentrated than gasoline for ignition to occur. Even when ignited, methanol burns about four times slower than gasoline, releases heat only at one-eighth the rate of gasoline fire, and is far less likely to spread to surrounding ignitable materials because of the low radiant heat output. It has been estimated by the EPA that switching from gasoline to methanol would reduce incidence of fuel-related fire by 90%. Methanol burns with a colorless flame, but additives can solve this problem. As methanol is completely miscible with water not only it is environmentally readily decomposed in nature but in contrast to ethanol there are no strict requirements needed to keep it dry to avoid phase separation from gasoline.
Methanol also provides an attractive and more environmentally-friendly alternative to diesel fuel. It does not produce smoke, soot, or particulates when combusted, in contrast to diesel fuel, which generally produces polluting particles during combustion. It also produce very low emissions of NO because it burns at a lower temperature than diesel. Furthermore, it has a significantly higher vapor pressure compared to diesel fuel, and the higher volatility allows easy start even in cold weather, without producing smoke typical of cold start with a conventional diesel engine. If desired, additives or ignition improvers, such as octyl nitrate, tetrahydrofurfuryl nitrate, peroxides or higher alkyl ethers, can be added to bring methanol's cetane rating to the level closer to diesel. Methanol is also used in the manufacture of biodiesel fuels by esterification of fatty acids.
As mentioned, the closely related derivative of methanol, which is a highly desirable alternative fuel, is dimethyl ether. Dimethyl ether (CH3OCH3), the simplest of all ethers, is a colorless, nontoxic, non-corrosive, non-carcinogenic and environmentally friendly chemical that is mainly used today as an aerosol propellant in spray cans, in place of the banned CFC gases. Dimethyl ether has a boiling point of −25° C., and is a gas under ambient conditions. Dimethyl ether is, however, easily handled as a liquid and stored in pressurized tanks, much like liquefied petroleum gas (LPG). The interest in dimethyl ether as alternative fuel lies in its high cetane rating of 55 to 60, which is much higher than that of methanol and is also higher than the cetane rating of 40 to 55 of conventional diesel fuels. The cetane rating indicates that dimethyl ether is effectively used in diesel engines. Advantageously, dimethyl ether, like methanol, is clean burning, and produces no soot particulates, black smoke or SO2, and only very low amounts of NOx and other emissions even without after-treatment of its exhaust gas. Some of the physical and chemical properties of DME, in comparison to diesel fuel, are shown in Table 1.
TABLE 1Comparison of the physical properties of DME and diesel fuelDimethylEtherDiesel FuelBoiling point ° C.−24.9180-360Vapor pressure at 20° C. (bar)5.1—Liquid density at 20° C. (kg/m)668840-890Heating value (kcal/kg)6,88010,150Cetane number55-6040-55Autoignition temperature (° C.)235200-300Flammability limits in air (vol %) 3-170.6-6.5
Currently, dimethyl ether is produced by the direct dehydration of methanol according to the following reaction:2CH3OH→CH3OCH3+H2O
Another methanol derivative is dimethyl carbonate (DMC), which can be obtained by converting methanol with phosgene or by oxidative carbonylation of methanol. DMC has a high cetane rating, and can be blended into diesel fuel in a concentration up to 10%, reducing fuel viscosity and improving emissions.
Methanol and its derivatives, e.g., dimethyl ether, DMC, and biodiesel fuel (esters of naturally occurring unsaturated acids) already have significant and expanding uses. They can be used, for example, as a substitute for gasoline and diesel fuel in ICE-powered cars with only minor modifications to the existing engines and fuel systems. Methanol can also be used in fuel cells, for fuel cell vehicles (FCVs), which are considered to be the best alternatives to ICEs in the transportation field. DME is also starting to be used in admixture to LNG and LPG in domestic and industrial fuel uses.
Methanol can also be used in reforming to produce hydrogen. In an effort to address the problems associated with hydrogen storage and distribution, suggestions have been made to use liquids rich in hydrogen such as gasoline or methanol as a source of hydrogen in vehicles via an on-board reformer. It was emphasized that methanol is the safest of all materials available for such hydrogen production. Further, because of the high hydrogen content of liquid methanol, even compared to pure cryogenic hydrogen (98.8 g of hydrogen in a liter of methanol at room temperature compared to 70.8 g in liquid hydrogen at about −253° C.), methanol is an excellent carrier of hydrogen fuel. The absence of C—C bonds in methanol, which are more difficult to break, facilitates its transformation to pure hydrogen in 80 to 90% efficiency.
In contrast to a pure hydrogen-based storage system, a reformer system is compact, containing on a volume basis more hydrogen than even liquid hydrogen, and is easy to store and handle without pressurization. A methanol steam reformer is also advantageous in allowing operation at a much lower temperature (250° C. to 350° C.) and for being better adapted to on-board applications. Furthermore, methanol contains no sulfur, a contaminant for fuel cells, and no nitrogen oxides are formed from a methanol reformer because of the lower operating temperature. Particulate matter and NOx emissions are virtually eliminated, and other emissions are minimal. Moreover, methanol allows refueling to be as quick and easy as with gasoline or diesel fuel. Thus, an on-board methanol reformer enables rapid and efficient delivery of hydrogen from liquid fuel that can be easily distributed and stored in the vehicle. To date, methanol is the only liquid fuel that has been demonstrated on a practical scale to be a suitable liquid fuel for a reformer to produce hydrogen for use in a fuel cells for transportation applications.
In addition to on-board reforming, methanol also enables convenient production of hydrogen in fueling stations for refueling hydrogen fuel cell vehicles. A fuel cell, an electrochemical device that converts free chemical energy of fuel directly into electrical energy, provides a highly efficient way of producing electricity via catalytic electrochemical oxidation. For example, hydrogen and oxygen (air) are combined in an electrochemical cell-like device to produce water and electricity. The process is clean, with water being the only byproduct. However, because hydrogen itself must first be produced in an energy-consuming process, by electrolysis or from a hydrocarbon source (fossil fuel) with a reformer, hydrogen fuel cells are still necessarily limited in their utility.
A system for producing high purity hydrogen has been developed by steam reforming of methanol with a highly active catalyst, which allows operation at a relatively low temperature (240° C. to 290° C.) and enables flexibility in operation as well as rapid start-up and stop. These methanol-to-hydrogen (MTH) units, ranging in production capacity from 50 to 4000 m3 H2 per hour, are already used in various industries, including the electronic, glass, ceramic, and food processing industries, and provide excellent reliability, prolonged life span, and minimal maintenance. As described above, operating at a relatively low temperature, the MTH process has a clear advantage over reforming of natural gas and other hydrocarbons which must be conducted at above 600° C., because less energy is needed to heat methanol to the appropriate reaction temperature.
The usefulness of methanol has led to the development of other reforming processes, for example, a process known as oxidative steam reforming, which combines steam reforming, partial oxidation of methanol, using novel catalyst systems. Oxidative steam reforming produces high purity hydrogen with zero or trace amounts of CO, at high methanol conversion and temperatures as low as 230° C. It has the advantage of being, contrary to steam reforming, an exothermic reaction, therefore, minimizing energy consumption. There is also auto thermal reforming of methanol, which combines steam reforming and partial oxidation of methanol in a specific ratio and addresses any drawback of an exothermic reaction by producing only enough energy to sustain itself. Auto thermal reforming is neither exothermic nor endothermic, and does not require any external heating once the reaction temperature is reached. Despite the aforementioned possibilities, hydrogen fuel cells must use highly volatile and flammable hydrogen or reformer systems.
U.S. Pat. No. 5,599,638 discloses a simple direct methanol fuel cell (DMFC) to address the disadvantages of hydrogen fuel cells. In contrast to a hydrogen fuel cell, the DMFC is not dependent on generation of hydrogen by processes such as electrolysis of water or reformation of natural gas or hydrocarbons. The DMFC is also more cost effective because methanol, as a liquid fuel, does not require cooling at ambient temperatures or costly high pressure infrastructure and can be used with existing storage and dispensing units, unlike hydrogen fuel, whose storage and distribution requires new infrastructure. Further, methanol has a relatively high theoretical volumetric energy density compared to other systems such as conventional batteries and the H2-PEM (PEM: proton exchange membrane) fuel cell. This is of great importance for small portable applications (cellular phones, laptop computers, etc.), for which small size and weight of energy unit is desired.
DMFC offers numerous benefits in various areas, including the transportation sector. By eliminating the need for a methanol steam reformer, DMFC significantly reduces the cost, complexity and weight of the vehicle, and improves fuel economy. A DMFC system is also comparable in its simplicity to a direct hydrogen fuel cell, without the cumbersome problems of on-board hydrogen storage or hydrogen producing reformers. Because only water and CO2 are emitted, emissions of other pollutants (e.g., NOx, particulate matter, SO2, etc.) are eliminated. Direct methanol fuel cell vehicles are expected to be of low emission (ZEV), and use of methanol fuel cell vehicles will greatly eliminate air pollutants from vehicles in the long term. Further, unlike internal combustion engine vehicles, the emission profile is expected to remain nearly unchanged over time. New fuel cell membranes based on hydrocarbon or hydrofluorocarbon materials with reduced cost and crossover characteristics have been developed that allow room temperature efficiency of about 34%.
Methanol and dimethyl ether provide a number of important advantages as transportation fuels. By contrast to hydrogen, methanol storage does not require any energy intensive procedures for pressurization or liquefaction. Because it is a liquid at room temperature, it can be easily handled, stored, distributed and carried in vehicles. It can act as an ideal hydrogen carrier for fuel cell vehicles through on-board methanol reformers or can be used directly in DMFC vehicles. Dimethyl ether although gaseous at room temperature can be easily stored under modest pressure and used effectively in admixture with diesel fuels and liquefied natural gas (LNG), or used in residential gas mixtures.
Methanol is also an attractive liquid fuel for static applications. For example, methanol can be used directly as fuel in gas turbines to generate electric power. Gas turbines typically use natural gas or light petroleum distillate fractions as fuel. Compared to such fuels, methanol can achieve higher power output and lower NOx emissions because of its lower flame temperature. Since methanol does not contain sulfur, SO2 emissions are also eliminated. Operation on methanol offers the same flexibility as on natural gas and distillate fuels, and can be performed with existing turbines, originally designed for natural gas or other fossil fuels, after relatively easy modification. Methanol is also an attractive fuel since fuel-grade methanol, with lower production cost than higher purity chemical-grade methanol, can be used in turbines. Because the size and weight of a fuel cell is of less importance in static applications than mobile applications, various fuel cells other than PEM fuel cells and DMFC, such as phosphoric acid, molten carbonate and solid oxide fuel cells (PAFC, MCFC, and SOFC, respectively), can also be used.
In addition to use as fuels, methanol, dimethyl ether and derived chemicals have significant applications in the chemical industry. Today, methanol is one of the most important feedstock in the chemical industry. The majority of the some 35 million tons of the annually produced methanol are used to manufacture a large variety of chemical products and materials, including basic chemicals such as formaldehyde, acetic acid, MTBE (although it is increasingly phased out for environmental reasons), as well as various polymers, paints, adhesives, construction materials, and others. Worldwide, methanol is used to produce formaldehyde (38%), methyl-tert-butyl ether (MTBE, 20%) and acetic acid (11%). Methanol is also a feedstock for chloromethanes, methylamines, methyl methacrylate, and dimethyl terephthalate, among others. These chemical intermediates are then processed to manufacture products such as paints, resins, adhesives, antifreeze, and plastics. Formaldehyde, produced in large quantities from methanol, is mainly used to prepare phenol-, urea- and melamine-formaldehyde and polyacetal resins as well as butanediol and methylene bis(4-phenyl isocyanate) MDI foam, which is used as insulation in refrigerators, doors, and in car dashboards and bumpers. Formaldehyde resins are predominantly used as adhesives in a wide variety of applications, e.g., manufacture of particle boards, plywood and other wood panels. Examples of major methanol-derived chemical products and materials produced are listed in FIG. 1.
In producing basic chemicals, raw material feedstock constitutes typically up to 60-70% of the manufacturing costs. The cost of feedstock therefore plays a significant economic role and its continued availability is essential. Because of its economic and long range availability advantages, methanol is considered a potential prime feedstock for processes currently utilizing more expensive feedstock such as ethylene and propylene, to produce chemicals including acetic acid, acetaldehyde, ethanol, ethylene glycol, styrene, and ethylbenzene, and various synthetic hydrocarbon products. For example, direct conversion of methanol to ethanol can be achieved using a rhodium-based catalyst, which has been found to promote the reductive carbonylation of methanol to acetaldehyde with selectivity close to 90%, and a ruthenium catalyst, which further reduces acetaldehyde to ethanol. Another feasible way to produce ethanol from methanol involves conversion of ethylene follow by hydration according to the overall reaction:2CH3OH→C2H5OH+H2O
Producing ethylene glycol via methanol oxidative coupling instead of using ethylene as feedstock is also pursued, and significant advances for synthesizing ethylene glycol from dimethyl ether, obtained by methanol dehydration, have also been made.
Conversion of methanol to olefins such as ethylene and propylene, also known as methanol to olefin (MTO) technology, is particularly promising considering the high demand for olefins, especially in polyolefin and synthetic hydrocarbon products production. The MTO technology is presently a two-step process, in which natural gas is converted to methanol via syn-gas and methanol is then transformed to olefin. It is considered that in the process, methanol is first dehydrated to dimethyl ether (DME), which then reacts to form ethylene and/or propylene. Small amounts of butenes, higher olefins, alkanes, and aromatics are also formed.

Various catalysts, include without limitation, synthetic aluminosilicate zeolite catalysts, such as ZSM-5 (a zeolite developed by Mobil), silicoaluminophosphate (SAPO) molecular sieves such as SAPO34 and SAPO-17 (UOP), as well as bi-functional supported acid-base catalysts such as tungsten oxide over alumina WO3/Al2O3, have been found to be active in converting methanol to ethylene and propylene at a temperature between 250° C. and 400° C. The nature and amount of the end product depend on the type of the catalyst, contact time and other factors of the process used. Depending on the operating conditions, the weight ratio of propylene to ethylene can be modified between about 0.77 and 1.33, allowing considerable flexibility. For example, when using SAPO-34 catalyst according to an MTO process developed by UOP and Norsk Hydro, methanol is converted to ethylene and propylene at more than 80% selectivity, and also to butene, a valuable starting material for a number of products, at about 10%. When using an MTO process developed by Lurgi with ZSM-5 catalysts, mostly propylene is produced at yields above 70%. A process developed by ExxonMobil, with ZSM-5 catalyst, produces hydrocarbons in the gasoline and/or distillate range at selectivity greater than 95%.
There is also a methanol to gasoline (MTG) process, in which medium-pore zeolites with considerable acidity, e.g., ZSM-5, are used as catalysts. In this process, methanol is first dehydrated to an equilibrium mixture of dimethyl ether, methanol and water over a catalyst, and this mixture is then converted to light olefins, primarily ethylene and propylene. The light olefins can undergo further transformations to higher olefins, C3-C6 alkanes, and C6-C10 aromatics such as toluene, xylenes, and trimethylbenzene.
With decreasing oil and natural gas reserves, it is inevitable that synthetic hydrocarbons would play a major role. Thus, methanol-based synthetic hydrocarbons and chemicals available through MTG and MTO processes are assuming increasing importance in replacing oil and gas-based materials. The listed uses of methanol in FIG. 1 are only illustrative and not limiting.
Methanol can also be used as a source of single cell proteins. A single cell protein (SCP) refers to a protein produced by a microorganism which degrades hydrocarbon substrates while gaining energy. The protein content depends on the type of microorganism, e.g., bacteria, yeast, mold, etc. The SCP has many uses, including uses as food and animal feed.
Considering the numerous uses of methanol and dimethyl ether, it is clearly desirable to have improved and efficient methods for their production. Currently, methanol is almost exclusively made from synthesis gas obtained from incomplete combustion (or catalytic reforming) of fossil fuel, mainly natural gas (methane) and coal.
Methanol can also be made from renewable biomass, but such methanol production also involves syn-gas and may not be energetically favorable and limited in terms of scale. As used herein, the term “biomass” includes any type of plant or animal material, i.e., materials produced by a life form, including wood and wood waste, agricultural crops and their waste byproducts, municipal solid waste, animal waste, aquatic plants, and algae. The method of transforming biomass to methanol is similar to the method of producing methanol from coal, and requires gasification of biomass to syn-gas, followed by methanol synthesis by the same processes used with fossil fuel. Use of biomass also presents other disadvantages, such as low energy density and high cost of collecting and transporting bulky biomass. Although recent improvements involving the use of “biocrude,” black liquid obtained from fast pyrolysis of biomass, is somewhat promising, more development is needed for commercial application of biocrude.
The presently existing methods of producing methanol involve syn-gas. Syn-gas is a mixture of hydrogen, carbon monoxide and carbon dioxide, and produces methanol over a heterogeneous catalyst according to the following reactions:CO+2H2→CH3OH ΔH298K=−21.7 kcal/molCO2+3H2→CH3OH+H2O ΔH298K=−9.8 kcal/molCO2+H2→CO+H2O ΔH298K=11.9 kcal/mol
The first two reactions are exothermic with heat of reaction equal to about 21.7 kcal·mol/l and about 9.8 kcal·mol/l, respectively, and result in a decrease in volume. Conversion to methanol is favored by increasing the pressure and decreasing the temperature according to Le Chatelier's principle. The third equation describes the endothermic reverse water gas shift reaction (RWGSR). Carbon monoxide produced in the third reaction can further react with hydrogen to produce methanol. The second reaction is simply the sum of the first and the third reactions. Each of these reactions is reversible, and is therefore limited by thermodynamic equilibrium under the reaction conditions, e.g., temperature, pressure and composition of the syn-gas.
Synthesis gas for methanol production can be obtained by reforming or partial oxidation of any carbonaceous material, such as coal, coke, natural gas, petroleum, heavy oil, and asphalt. The composition of syn-gas is generally characterized by the stoichiometric number S, corresponding to the reaction shown below.
  S  =            (                        moles          ⁢                                          ⁢                      H            2                          -                  moles          ⁢                                          ⁢                      CO            2                              )              (                        moles          ⁢                                          ⁢          CO                +                  moles          ⁢                                          ⁢                      CO            2                              )      Ideally, S should be equal to or slightly above 2. A value above 2 indicates excess hydrogen, while a value below 2 indicates relative hydrogen deficiency. Reforming of feedstock having a higher H/C ratio, such as propane, butane or naphthas, leads to S values in the vicinity of 2, ideal for the conversion to methanol. When coal is used, however, additional treatment is required to obtain an optimal S value. Synthesis gas from coal requires treatment to avoid formation of undesired byproducts.
The most widely used technology to produce syn-gas for methanol synthesis is steam reforming. In this process, natural gas (of which methane is the major component) is reacted in a highly endothermic reaction with steam over a catalyst, typically based on nickel, at a temperature of about 800° C. to about 1,000° C., and a pressure of about 20 atm to 30 atm) to form CO and H2. A part of the CO formed reacts consequently with steam in the water gas shift reaction (WGS) to yield more H2 and also CO2. The gas obtained is thus a mixture of H2, CO and CO2 in various concentrations depending on the reaction conditions, such as temperature, pressure and H2O/CH4 ratio according to the following reactions:CH4+H2O→CO+3H2 ΔH298K=49.1 kcal/molCO+H2O→CO2+H2 ΔH298K=−9.8 kcal/mol
Since the overall methane steam reforming process is highly endothermic, heat must be supplied to the system by burning a part of the natural gas used as the feedstock. The stoichiometric number S obtained by steam reforming of methane is close to 3, much higher than the desired value of 2. This can generally be corrected by addition of CO2 to the steam reformer's exit gas or use of excess hydrogen in some other process such as ammonia synthesis. However, natural gas is still the preferred feedstock for methanol production because it offers high hydrogen content and, additionally, the lowest energy consumption, capital investment and operating costs. Natural gas also contains fewer impurities such as sulfur, halogenated compounds, and metals which may poison the catalysts used in the process.
The existing processes invariably employ extremely active and selective copper-based catalysts, differing only in the reactor design and catalyst arrangement. Because only part of the syn-gas is converted to methanol after passing over the catalyst, the remaining syn-gas is recycled after separation of methanol and water. There is also a more recently developed liquid phase process for methanol production, during which the syn-gas is bubbled into the reaction mixture. Although the existing processes have methanol selectivity greater than 99% and energy efficiency above 70%, crude methanol leaving the reactor still contains water and other impurities, such as dissolved gases (e.g., methane, CO, and CO2), dimethyl ether, methyl formate, acetone, higher alcohols (ethanol, propanol, butanol), and long-chain hydrocarbons. Commercially, methanol is available in three grades of purity: fuel grade, “A” grade, generally used as a solvent, and “AA” or chemical grade. Chemical grade has the highest purity with a methanol content exceeding 99.85% and is the standard generally observed in the industry for methanol production. The syn-gas generation and purification steps are critical to the existing processes, and the end result would largely depend on the nature and purity of the feedstock. To achieve the desired level of purity, methanol produced by the existing processes is usually purified by sufficient distillation. Another major disadvantage of the existing process for producing methanol through syn-gas is the energy requirement of the first highly endothermic steam reforming step. The process is also inefficient because it involves transformation of methane in an oxidative reaction to CO (and some CO2), which in turn must be reduced to methanol.
Another way to produce syn-gas from methane is through the partial oxidation reaction with insufficient oxygen, which can be performed with or without a catalyst. This reaction is exothermic and is conducted at high temperature of about 1,200° C. to about 1,500° C. The problem with partial oxidation is that the products, CO and H2 are readily further oxidized to form undesired CO2 and water in highly exothermic reactions leading to S values typically well below 2 and contributing to CO2 induced global warming. The following reactions are illustrative of the process.CH4+½O2→CO+2H2 ΔH298K=−8.6 kcal/molCO+½O2→CO2 ΔH298K=−67.6 kcal/mol
To produce syn-gas without either consuming or producing much heat, modern plants are usually combining exothermic partial oxidation with endothermic steam reforming in order to have an overall thermodynamically neutral reaction while obtaining a syn-gas with a composition suited for methanol synthesis (S close to 2). In this process, called autothermal reforming, heat-produced by the exothermic partial oxidation is consumed by the endothermic steam reforming reaction. Partial oxidation and steam reforming can be conducted separately or simultaneously in the same reactor by reacting methane with a mixture of steam and oxygen. The process as mentioned however, produces large amounts of CO2 necessitating its costly sequestering or venting into the atmosphere. Any carbon containing fuel or derived synthetic hydrocarbon product when oxidatively used results in the formation of carbon dioxide and thus is not renewable on the human time scale. There is an essential need to make carbon fuels renewable and thus also environmentally neutral to minimize their harmful effect on global warming.
The selective conversion and recycling of carbon dioxide to methanol without generating unwanted by-products is thus a major challenge and a much desired practical goal. There is a great need to effectively and economically produce methanol from carbon dioxide with high selectivity and yield of conversion.